User's Guide

2110059 Rev 1.1 Proprietary and Confidential Page 29
5. CDPD Operation
This section provides information on configuring and using the modem in CDPD mode if you are
not using Watcher and Windows Dial-up Networking for your connections. If you are using a
standard Windows-based application then consult Section 3.3 Configuration using WirelessExpert.
5.1. Cellular Digital Packet Data (CDPD) Introduction
CDPD is a wireless radio frequency (RF) packet switched data communications service which
provides two-way, fully digital, 19.2 kbps packet data networking services to mobile hosts. All
data is exchanged with the network in the form of IP packets (TCP or UDP). It is simply a
wireless extension of traditional networks, providing the user with seamless access to data
applications on these networks.
Instead of using a dedicated cellular channel or telephone line to transmit data, CDPD devices,
like the Sierra Wireless modem, are able to send data over cellular voice channels (the airlink)
during the time periods when these radio channels are not used for standard cellular voice
communications. When a channel becomes overloaded or used by a cellular voice call, the CDPD
devices “hop” to an unused or idle channel. Since this initial design implementation, many areas
now dedicate specific channels to only CDPD traffic.
With multiple mobile devices sharing the cellular channel, data transfer rates can vary depending
on channel loading. Maximum data rates in the order of 1000 characters per second (10 kbps) are
achievable only under lightly loaded ideal conditions.
The cellular radio channel is only used when actual data is being sent or received and not when the
user application is idle. This contrasts with CSC mode which “owns” the connection channel as
long as the connection is open. CSC consumes the cellular resource even when the application is
not actively communicating.
CDPD provides the following airlink support:
• Compression of packet header and information fields transmitted over the wireless channel to
reduce the amount of the airlink resource used.
• Support of many users on the same cellular RF channel at the same time.
• Error correction of data sent over the airlink.
• Movement of the user from one cell site to another.
• Delivery of properly sequenced data between user applications over the airlink.
The CDPD network operates as a collection of CDPD service provider networks, with the service
provider networks being operated by cellular carriers who provide services such as:
• Data connection to other networks
• Application services
• Network management
• Network security
• Accounting and billing
5.1.1. Security
With data exchanged over a broadcast radio link, security is required to prevent eavesdropping.
Airlink security is automatically provided by encrypting the data packets between the SB300
Series modem and the service provider’s Mobile Data Intermediate System (where the radio signal
is routed to a wireline public or private data network). If end-to-end data security is desired it
must be implemented or otherwise provided by the end-user.